The invention relates in general to generating holes in composites, and in particular, to generating countersunk holes in composites comprising at least two materials having different moduli of elasticity with a combination drill/countersink tool.
In the past, it has been difficult to quickly and accurately drill holes in composite material, such as, for example, aramid fiber/epoxy or other resin-bonded laminates. Experience has shown that the aramid fibers, which have a smaller diameter and a higher tensile strength, modulus of elasticity, and shear resistance than the more conventional glass fibers used in similar composite compositions, tend to deflect and bend before being cut by the shearing action of the cutting edge of the drill. This action forces the fibers into the softer resilient resin matrix material, increasing radial compressive forces on the drill and the surrounding composite material. These compressive forces generate frictional heat which further softens the resin matrix, by allowing the rough aramid fiber to further escape the shearing action of conventional drills. The heat generated also limits the drill speed and feed pressure so that the material surrounding the drill hole is not damaged by excessive heat. Also, as a result of this characteristic of aramid fiber reinforced resin laminates, holes drilled by conventional means are often undersized holes with frayed, fuzzy edges, and with mushrooming of the aramid fibers occurring at the tool exit.
The above-referenced U.S. patent application Ser. No. 166,868 disclosed an optimized method and tool for quickly and easily generating both shallow and deep holes with clean hole edges in composites of materials with different moduli of elasticity.
The method provides for simultaneous point and surface cutting. Point and surface cutting occur at the outer circumferential surface of the hole and generally along a radius of the hole to be drilled. At the outer circumferential surface, the point cutting proceeds normal to the planar area of the hole, while the surface cutting proceeds circumferentially.
The tool, which is generally shaped as a cylindrical rod, has an axially extending flute and an end surface which is inclined to the tool axis. The intersection of the end surface with one side surface of the flute, the intersection of the end surface with the semi-cylindrical outer surface of the rod, and the intersection of the one end surface of the flute and semi-cylindrical outer surface of the rod define a cutting angle end point and associated cutting edges. The end surface intersects the one side of the flute adjacent the end point at an acute angle to form a first cutting edge. The same side of the flute intersects the outer surface adjacent the end point to form the second cutting edge. The second cutting edge intersects the first cutting edge at an acute angle at the single end point. The end surface also intersects the semi-cylindrical outer surface to form a third edge which intersects the first and second cutting edges at the single end point at respective acute angles.
During a hole forming operation, the single end point first cuts the composite and thereafter penetrates successive transverse planes of the composite, i.e., transverse to the longitudinal axis of the hole to be generated, and the portions of the cutting edges immediately adjacent the single end point cleanly cuts the fibers of the composite of each successively cut transverse plane in the immediate region of penetration. Penetration is facilitated by the two cutting edges and the third edge adjacent the single end point. The first cutting edge also cuts away chips from the cylindrical composite portion to be removed to form the hole progressively inward. By penetrating the composite and severing the fibers in the immediate region of the penetration, very little deflection of these fibers occurs; thus, the radial compressive forces exerted on the tool by these fibers and the thrust requirements of the tool are reduced to a minimum, and an accurately sized, clean-cut hole is produced.
In some applications of the composite material described above, it is necessary to form countersunk holes therein, that is, holes having enlarged entrance portions for receiving the heads of screws or bolts inserted therein, in order to satisfy flush installation requirements.